Abstract: Apparatus and methods for flushing and cleaning the oil strainer and crankcase of an internal combustion engine of a type having a removable full-flow oil filter. The oil filter is temporarily replaced with a cartridge which has at least one passage therethrough which will communicate with the discharge side of the engine oil pump but does not communicate with the normal outflow port of the filter apparatus. A second passage therethrough may communicate with the outflow port of the filter system but does not communicate with the discharge of the oil pump. In a preferred method, the inlet of the first passage may be connected to a source of pressurized fluid to force the fluid, in a direction opposite to normal flow, through the passage, the oil pump and the oil strainer to agitate and flush contaminants in the oil strainer and crankcase. The pressurized fluid and contaminants are drained from the crankcase after which the cartridge is removed, a new oil filter installed and clean oil added to the engine.